The present invention relates to a pneumatic tire, more particularly, to a bead and lower-sidewall structure being capable of reducing the tire weight without deteriorating tire performance.
In recent years, from a point of view of environmental protection, it has become very important for automobile tires or pneumatic tires to reduce tire weight. The tire weight can be easily decreased by decreasing the thicknesses of rubber components and the ply number of the carcass. But, there is a possibility that the steering stability will deteriorate by the resultant decrease in tire rigidity. If rubber thickness, for example the sidewall rubber thickness, is decreased, then the vibration increases due to resonance of the air in the tire whose fundamental frequency is about 200 to 250 Hz. This becomes a problem for the recent silent high-grade passenger cars.
In the published Japanese patent application JP-B2-1-33363, a radial tire for passenger cars is disclosed, wherein, the radial carcass comprises at least one ply of nylon or polyester or rayon cords, and as shown in FIG. 5, the bead portion is provided between a carcass ply turned up portion (c1) and a rubber filler (b) with a reinforcing layer (a) of aromatic polyamide or metallic cords arranged at an angle of from 45 to 75 degrees with respect to the carcass cords. The reinforcing layer (a) extends from the bead base to a radial height of from 50 to 75% of the tire section height. The radially outer end of the rubber filler (b) which has a dynamic elastic modulus of at least 300 kg/sq.cm, reaches to a radial height of 30 to 70% of the tire section height. The objective of such structure is to improve the steering stability by effectively reinforcing the bead portion and sidewall portion. In such a tire, as the bead portion and sidewall portion are reinforced by the relatively large-sized rubber filler and reinforcing layer, a considerable weight increase is inevitable.
It is therefore, an object of the present invention to provide a pneumatic tire in which the tire weight is reduced without deteriorating other performance such as steering stability, noise performance and the like.
According to the present invention, a pneumatic tire comprises
a tread portion,
a pair of sidewall portions,
a pair of bead portions each with a bead core and a bead apex therein,
a carcass comprising a ply of cords extending between the bead portions through the tread portion and sidewall portions and turned up around the bead core in each bead portion from the inside to the outside of the tire so as to form a pair of turned up portions and a main portion therebetween,
the bead apex made of hard rubber disposed between the main portion and turned up portion and extending radially outwards from the bead core, a length (LA) of the bead apex between the radially inner end and radially outer end thereof being in a range of from 0.1 to 0.25 times the section height (H) of the tire,
a reinforcing cord layer disposed along the axially inside of each turned up portion, the reinforcing cord layer having a radially outer end (FU) which is positioned radially outside the radially outer end (BU) of the bead apex but radially inside the maximum tire section width point (M), and a radially inner end (FD) which is positioned radially outside the radially outer end of the bead core but radially inside the radially outer end (BU) of the bead apex,
a length (LB) of the reinforcing cord layer between the radially inner end and the radially outer end thereof being in a range of from 1.2 to 2.0 times the length LA of the bead apex,
the sidewall portions having a minimum thickness (Wmin) being in a range of not more than 0.5 times a maximum thickness (Wmax) of a region where the reinforcing cord layer exists.
An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings.